2. MAGNETIC FIELD STRUCTURE ENVIRONMENT IN THE MILKY WAY

The likely global structure of the magnetic field in
our Galaxy can be gleaned from two examples, shown below,
Messier 51 (NGC 5194) seen face-on, and NGC 891, an
edge-on example. Both were
imaged at more than one radio frequency, so that the local projected
plane of polarization is corrected for Faraday rotation
(2) to its
intrinsic orientation. Figure 1 shows a large
scale ordered magnetic
field component that is approximately parallel to the spiral arms. The
edge-on image of NGC 891 in Figure 2 gives a
projected view of the
above-plane magnetic structure, which has a characteristic X-shape. The
magnetic field lines are directed vertically outward above the nuclear
zone, and gradually become more parallel to the disk at larger
galactocentric radii.

Figure 1.NGC 891 imaged with the Effelsberg
radio telescope, with Faraday RM-corrected, projected magnetic field
orientations
[1].
By permission of R. Beck.

Figure 2. Face-on galaxy M51, showing an
overlay of the optical image, radio contours, and the RM-corrected
magnetic field orientations as in Fig. 1.
[2].
Reproduced from Sterne und Weltraum by permission of R. Beck.

To calculate the HECR propagation paths to us, especially below ~
1018 eV, we need to know the 3-D magnetic structure of the
Milky Way, and any superimposed magnetic turbulence. The 2-D images in
Figs 1 and 2 give important clues on the Milky Way's projected global
field pattern. Galactic dynamo calculations have predicted either a
dipole or quadrupole field structure, and these have been so far been
difficult to verify observationally in any nearby galaxy, including the
Milky Way. There is some evidence to show that neither a pure dipole or
quadrupole give an ideal fit to the data, but one or the other seems to
give a good approximation in some restricted investigations. The 3-D
off-plane structure of the Milky halo has been difficult to discern
observationally because there is relatively little Faraday rotation at
high Galactic latitudes
(b), hence at high z-heights above the plane. Models for
our Galaxy's magnetic structure are best based on an all-sky
distribution of the RM's of extragalactic radio sources, and/or of
(mostly Galactic) pulsars. Pulsar RM's to date have been smaller in
number than their e.g.r.s. counterparts, and their RM pathlengths
through the ISM cover only part of the extragalactic source
pathlength. An advantage of pulsar magnetic probes is that the pulsar
dispersion measures (DM) can be used, via an ISM model of
ne, to model the RM path length through the
Milky Way disk. Future larger numbers of RM and DM for the pulsars may
lead to the most definitive 3-D Galactic disk field models for
propagation paths and deflections of HE charged particles.

Meanwhile, recent progress has been made with ever larger numbers of
extragalactic source RM's, as is illustrated in a recent all-sky plot of
ca. 2250 rotation measures from a recent compilation made in 2009
[3].
A 3-panel plot in Figure 4 shows the clearest
demonstration yet for an underlying magnetic field in our region of the
Milky Way disk. This brings us to the most immediately relevant zone for
calculating corrected UHECR arrival directions from outside the Galaxy.

The smoothed RM plot in Figure 4 is derived from
1500 newly determined RM's, plus ca. 750 additional RM's
published in the literature. The 1500 set are an extension of the 555 RM
set of Simard-Normandin, Kronberg & Button and were determined by
the same 7-step procedure described in that paper
[5].
They were then smoothed using a procedure developed by Simard-Normandin
& Kronberg
[4],
which iteratively tests for, and rejects RM outliers, and/or unreliable
RM values. These include genuinely anomalous RM's that deviate from the
"RM consensus" in a given (l, b) direction.

Using this new compilation, Kronberg & Newton-McGee
[3]
produced plots, shown in Figure 5, of similarly
smoothed RM's for different subsets of only low Galactic latitude RM's -
in this plot only for RM's at latitudes |b|
15°. The smoothing
full width was close to 20°, and this is near to ideal for
"filtering out" an underlying coherent component of the Galactic disk
field. These results, together with the higher latitude RM's in
Figure 3
are being used with other input to construct propagation models to match
with UHECR observations.

Figure 4. (a) Variation of RM's over the
Galactic plane, when smoothed with a resolution roughly comparable with
the width and (height) of a spiral arm. Black and orange identify RM's
on either side of l = 0°. (b) The orange points are folded
about l = 0°, and their RM sign is reversed (i.e. fold,
reverse). (c) The difference between the curves in (a),(b) at
|l| 90° for a
range of shift angles was minimized to obtain a least squares
minimized, optimum relative shift of 11° ± 2° (fold,
reverse, shift).

Another all-sky investigation has used the 408 MHz all sky continuum
synchrotron radiation survey of Haslam et al
[6]
and a more recent 1.4 GHz survey by Reich & Reich
[7].
Breuermann et al
[8]
and more recently Berkhuijsen
[9]
have combined these data with various modeling
assumptions to estimate the variation of total magnetic field strength,
|Bt|, with Galactocentric radius in the disk
plane. This is shown in Figure 5. It shows an
approximately exponential decay form

(1)

where R0B = 11 ± 0.4 kpc. We note from
Fig. 5 that |Bt| remains a
surprisingly strong 4 µG at R = 17 kpc. The results
in Fig. 5 give good overall agreement with
similar results from Broadbent et al.
[10]
and more recent study by Strong et al.
[11].
The latter are based on
-ray
emission and, notably, they are independent of the CR/magnetic energy
equipartition assumptions needed in the radio sky analyses.

These results lead us into the less determined metagalaxy zone, or
interface to the magnetic structure of the IGM in the local Universe
(Zone (5))

Figure 5. The variation of modeled
interstellar magnetic field strength, |Bt|, as a
function of Galacto-centric radius out to 17 kpc from Berkhuijsen
[9].
See text for further details. Reproduced with permission from Elly
Berkhuijsen.